1. Field of the Invention
The present invention relates to a semiconductor device and a semiconductor device manufacturing method, and more particularly to a semiconductor device capable of fixing a potential at a surface portion of a semiconductor layer located outside an active region and a semiconductor device manufacturing method capable of manufacturing the semiconductor device.
2. Description of the Background Art
Higher efficiency and lower loss of a power device used for conversion or control of electric energy have recently be demanded. For a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) serving as a power switching device, lowering in ON resistance or the like by improving an element structure or selection of a material has been studied, and for example, a trench (groove) type element structure instead of a planar (plane) type element structure has increasingly been adopted. The trench type element structure refers to an element structure characterized by formation of a channel region along a wall surface of the trench formed in a surface of an element.
Since a MOSFET serving as a power switching device is used for conversion, control, or the like of a high voltage, it is required to achieve high efficiency and low loss as well as a high breakdown voltage. With a trench type element structure, lower loss of an element owing to lowering in ON resistance is achieved, while a breakdown voltage of an element may lower due to a defective shape of a formed trench or the like. Specifically, local electric field concentration occurs in a portion where a defective shape of a trench is present, and resulting high electric field is likely to break a gate insulating film formed on a wall surface of the trench. In order to suppress such lowering in breakdown voltage of an element due to a trench structure, for example, a MOSFET adopting such a structure that a region to serve as an active region of an element is surrounded by a trench different from the trench in the active region has been proposed (see, for example, Japanese Patent Laying-Open No. 2005-322949).
In the MOSFET proposed in PTL 1, by causing a trench surrounding an active region to function as an electric field relaxing portion, a breakdown voltage of an element can be improved. With this MOSFET, however, for example, when a high voltage of approximately 1 kV (kilovolt) is applied, it is difficult to exhibit a sufficient electric field relaxing function. Therefore, in order to obtain resistance of an element against application of such a high voltage, further improvement in element structure is demanded.